There are a number of references relating to the treatment of stainless steel for use in pyrolysis processes. One of the leading researchers is Professor L. Albright at the University of Purdue. The summary of the Ph.D. thesis by Ta-Chi Luan "Reduction of Coke Deposition in Ethylene Furnaces" published August 1993 discloses treating various alloys with mixtures of hydrogen and water. The ratio of hydrogen to water appears to be about 50:1 (pages 16 and 17) which is greater than that contemplated in the present invention.
U.S. Pat. No. 5,169,515, issued Dec. 12, 1992, assigned to Shell, teaches the treatment of stainless steel furnace tubes at temperatures from 1800.degree., preferably from 1900.degree. to 2200.degree. F. (about 1,000.degree.-1200.degree. C.) with hydrogen and steam in a ratio from 0.05 to 5 and in the presence of from about 100 to 500 ppm of hydrogen sulphide or a compound which generates hydrogen sulphide. The Shell patent teaches that the steam (or water) must be present in an amount of about 5 weight % to 500 weight % of the hydrogen. The present process is directed to a process using significantly lower amounts of steam.
U.S. Pat. No. 5,288,345, issued Feb. 22, 1994 to NKG Insulators Inc., discloses treating a sintered alloy containing aluminum at a temperature from 800.degree. to 1300.degree., preferably from 1,000.degree. to 1200.degree. C. in an atmosphere which contains water in an amount corresponding to a dew point of from 30.degree. to 60.degree. C. This is a larger amount of water than required by the present inventors. Further, the object of the treatment is to reduce oxidation and there is no reference in the disclosure to carburization.
U.S. Pat. No. 4,424,083, issued Jan. 3, 1984 to Exxon Research and Engineering Corporation, discloses shot peened stainless steel in an atmosphere containing hydrogen and steam having a dew point of 60.degree. C. Again, this is a higher amount of hydrogen than specified in the present patent application.
All of the above art teaches treating stainless steel with hydrogen or an inert atmosphere such as nitrogen containing relatively high amounts of an oxidizing gas such as water or steam, in amounts of at least 2%. The present invention is directed to treating stainless steel with an inert atmosphere containing smaller amounts of an oxidizing gas.